The maximum operating speed of digitizers establishes a limit for many systems on the maximum frequency bandwidth of a signal that can be digitized; however, once digitized, processing of the signal may proceed at whatever rate is appropriate.
It will be appreciated by those skilled in the art that any signal may be digitized and the resulting digital representation of the signal used in a variety of signal processing algorithms for various purposes. For example, digital signal processing algorithms exist for the filtering of a signal and the demodulation of a signal. Further, in order for a digitizer to properly represent a signal in its digitized form as a sampled low pass signal, the digitizer must operate at a sampling rate at least equal to twice the highest frequency component of the signal. Similarly, the digitizer must sample at least twice the maximum bandwidth of the signal, if it is a bandpass signal. This minimum sampling rate is also known as the Nyquist sampling rate. By digitizing a signal at the Nyquist sampling rate, the signal is assured of being able to be represented uniquely in a digitized form without aliasing of the signal frequency components. When such a unique representation is provided, the signal may be completely reconstructed into its original form without loss of information. Otherwise if a slower rate of sampling is used on a signal, false information may be included in the sampled signal information. This false information is caused by aliasing (also known as fold-over) which occurs when a high-frequency component in the spectrum of a signal being sampled apparently takes on the identity of a lower frequency in the spectrum of a sampled version of the signal.
Digitizing a signal can be conveniently performed by an analog-to-digital converter which is preceded by an appropriate anti-aliasing filter. The combination of these two signal processing elements are typically referred to as a digitizer. A digitizer accepts an input analog signal that is first filtered to restrict the bandwidth of the signal to prevent aliasing during a subsequent sampling process. The filtered signal is then sampled to generate a digital representation of the input signal's amplitude at different points in time at a rate which is generally determined by a system sampling clock. When the sampling clock occurs at a periodic rate, the signal is said to be sampled at a uniform rate. Other techniques such as sampling at non-uniform rates, where the sampling may not occur at regular intervals in time, also exist; however, the extension of the following inventive concepts from uniform to non-uniform sampling rates would be understood by those skilled in the art.
Digitizers are characterized by a variety of specifications, typically the maximum sampling rate in samples per second and the number of bits of resolution which can be generated. The sampling rate must be at least equal to the Nyquist rate and preferably higher than this to ease the anti-aliasing filtering requirements of the digitizer. An anti-aliasing filter is placed ahead of the digitizer to limit the bandwidth of the signal (i.e., attenuate the out-of-band high frequency signal components) such that the Nyquist criterion is met with a given sampling rate. Additionally, it is generally desirable to provide the maximum number of bits of resolution possible so the signal can be accurately digitized with the least amount of quantization error. Quantization error can occur when sampled values of a continuous message signal are rounded off to the nearest representation level. Unfortunately, a high number of bits of resolution and a high sampling rate are contradictory design goals and compromises often must be made in the digitizer design process.
A variety of techniques are available to sample wide frequency bandwidth signals. One known technique is to simply increase the sampling rate to very high values, often measured in hundreds of Megasamples per second. Unfortunately, this wide bandwidth sampling is achieved at the expense of consuming greater levels of power and providing lower resolution than typically desired. The digitizers generally have high power dissipation levels, because, even if the devices are fabricated with low power Complementary Metal Oxide Semiconductor (CMOS) technology, the devices have a power dissipation which is proportional to the operating speed of the circuit. In addition, resolution also may suffer at the higher sampling rates due to inaccuracies generated in the analog-to-digital conversion process and the limitations on circuit operating speed. The highest rate digitizers are also expensive to manufacture and sometimes require extensive manual adjustments for optimum performance. Due to these design constraints, it will be appreciated by those skilled in the art that even with the fastest digitizers available, the maximum processing rate of a signal processing system may be limited not by the digital signal processing elements of the system but by the digitizer itself.
Another known technique samples repetitive signals through a relatively slow, but repetitive, random sampling process to synthesize the effect of a much higher sampling rate. This technique fails to provide an accurate digitized representation of an input signal when the signal to be digitized is not repetitive and is instead random or stochastic in nature. Wide bandwidth stochastic signals tend to be a significant portion of a typical signal communication. In some instances, the wide bandwidth stochastic signals are more common than repetitive signals. Therefore, a need exists for a wide bandwidth digitizer which utilizes lower power dissipation devices, which has a higher resolution, and which more accurately represents an input signal.